The concentration of carbon in living matter (18%) is almost 100 times greater than its concentration in the earth (0.19%). So living things extract carbon from their nonliving environment. For life to continue, this carbon must be recycled. That is our topic.

Carbon exists in the nonliving environment as:

carbon dioxide (CO2) in the atmosphere and dissolved in water (forming HCO3−)

The carbon dioxide content of the atmosphere is gradually and steadily increasing. The graph shows the CO2 concentration at the summit of Mauna Loa in Hawaii from 1958 through 1999. The values are in parts per million (ppm). The seasonal fluctuation is caused by the increased uptake of CO2 by plants in the summer. (In April 2018, its average concentration at Mauna Loa reached a record-breaking 410 ppm.)

The increase in CO2 probably began with the start of the industrial revolution. Samples of air trapped over the centuries in the glacial ice of Greenland show no change in CO2 content until 300 years ago.

Since measurements of atmospheric CO2 began late in the nineteenth century, its concentration has risen over 20%. This increase is surely "anthropogenic"; that is, caused by human activities:

burning fossil fuels (coal, oil, natural gas) which returns to the atmosphere carbon that has been locked within the earth for millions of years.

clearing and burning of forests, especially in the tropics. In recent decades, large areas of the Amazon rain forest have been cleared for agriculture and cattle grazing.

Despite these "sinks" for our greatly-increased CO2 production, the concentration of atmospheric CO2 continues to rise? Should we be worried?

Carbon dioxide is transparent to light but rather opaque to heat rays. Therefore, CO2 in the atmosphere retards the radiation of heat from the earth back into space — the "greenhouse effect".

Has the increase in carbon dioxide led to global warming?

Some evidence:

Careful monitoring shows that the global air temperature in 2014 was 0.57°C higher than the average from 1961–1990, and that 14 of the 15 warmest years since records began being kept late in the 19th Century have occurred in this century (including 2005 and 2010 as well as 2014).

Many glaciers and ice sheets are receding.

Woody shrubs are now growing in areas of northern Alaska that 50 years ago were barren tundra.

Many angiosperms in temperate climates are flowering earlier in the spring than they used to.

Many species of birds and butterflies are moving north and breeding earlier in the spring.

Will continued increase in carbon dioxide lead to more global warming and, if so, how much?

At this point, the answer depends on what assumptions you plug into your computer models. But as the different models have been improved, they seem to be converging on a consensus: a doubling of the CO2 concentration (expected by the end of this century) will cause the earth to warm somewhere in the range of 1.1–6.4°C.

release from the GI tract of the cattle that are later placed on the cleared land.

The methane concentration in Arctic air is presently some 1.9 parts per million, the highest level seen such measurements began.
Although this concentration is far less than that of CO2, methane is 28–34 times as potent a greenhouse gas.

The marked warming of the earth that occurred at the end of the Paleocene epoch is thought to have been caused by the release of large amounts of methane from the sea floor.